The present disclosure relates to permanent magnet motors. More particularly, the present disclosure relates to a permanent magnet motor including a plurality of rotor modules and a pump assembly including an electric submersible pump driven by a permanent magnet motor.
Permanent magnet machines such as permanent magnet motors or generators have been widely used in a variety of applications including aircraft, automobile, subsea and industrial usage. A requirement for lightweight and high power density permanent magnet machines has resulted in the design of higher speed motors and generators to maximize the power to weight ratios. Hence, the trend is increasing acceptance of permanent magnet machines offering high machine speed, high power density, reduced mass and cost.
Permanent magnet motors typically employ permanent magnets either in the rotor, the stator or both. In most instances, the permanent magnets are found within the rotor assembly. The output power of the permanent magnet motor is determined by the length of the stator and rotor assemblies. A longer axial length corresponds to increased power.
In one application, permanent magnet machines, and more particularly, permanent magnet motors may be used to drive an electric submersible pump. Currently, the majority of electric submersible pumps are driven by induction motors. Permanent magnet motors can provide a larger torque such that the number of motors needed to drive the electric submersible pump may be reduced, therefore decreasing the overall system cost and complexity. The use of permanent magnet motors also enables efficient pump operation at higher speeds, thus increasing the pumping capacity. In addition, the efficiency of the permanent magnet motor is higher than the induction alternative so that overall system efficiency is increased.
In conventional permanent magnet machines, multiple permanent magnets are positioned as either internal magnets, and more particularly embedded inside multiple laminations of a rotor core, or surface mounted on an exterior portion of the rotor core. With internally positioned magnets, the mechanical stresses in the rotor are concentrated in multiple bridges and center posts. For higher speed applications, the thickness of the multiple bridges and center posts have to be increased for enhanced structural strength of the rotor and various other parts. The increased thickness leads to more magnet flux leakage into the multiple bridges and center posts, which leakage significantly reduces the machine power density, resulting in decreased efficiency of the machine. With surface mounted magnets, magnets are positioned on an exterior portion of the rotor. Hence, the magnets need some retention system against centrifugal forces occurring during rotation. A retaining sleeve is often used for this purpose, but the introduction of a retaining sleeve increases the effective magnetic airgap and introduces additional eddy current losses if the sleeve is metallic. This significantly reduces the machine power density, resulting in decreased efficiency of the machine.
Therefore, it is desirable to have a permanent magnet machine with high power density and efficiency. In addition, it is desirable to provide a permanent magnet machine for driving an electric submersible pump.